Agree with others. Either 1 or more of the brakes are not working on 1 side, or not adjusted properly.
You don't feel the pull at lower settings because the functioning brakes are not working as hard.
BTW, I set my controller down instead of up when entering stop and go traffic. That keeps the trailer brakes from grabbing during the constant stopping. When back on the road, I will again set the controller back up for more power to the brakes......it is harder to stop the trailer at 65 mph than at 25 mph.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263